For a problem I was noodling with in R, I needed to generate random trees (meaning connected, acyclic, layered, undirected graphs, not classification trees or anything statistical). There are a bunch of libraries for R that have various capabilities for constructing and or massaging graphs and networks (see the CRAN Task View for graphical models). After spending some time poking through the docs for some of the libraries listed in the task view, and unsuccessfully trying to install one, I decided it would be faster just to roll my own code.
Along the way I succumbed to the software developer's maxim: if it works, it needs more features. Still, I wound up with a function that is not horribly complicated and seems to work. Given how many nodes and how many layers you want in the tree, it outputs a matrix of edges forming the desired tree. There are optional arguments for how you want the nodes labeled (the default is 1, 2, ... but you can supply a vector of labels) and in what order the labels should be assigned (top to bottom/left to right raster scan of the graph or randomly), as well as how you want the output matrix organized (randomly, by label order, or by layer order).
The code is in an R notebook that demonstrates the use of the function. It imports the DiagrammeR library in order to plot the resulting trees, but the function does not require DiagrammeR and in fact has no dependencies. You can view the notebook here, and if you want you can download the code from it (see the select box in the upper right of the notebook). The code is licensed under the Creative Commons Attribution 3.0 Unported license.
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